Cancer is the second leading cause of death in the United States (U.S.). In 1999 there were an estimated 563,100 cancer deaths and each year about 1,222,000 new cancer cases are diagnosed. Among these, solid tumor cancers such as lung, breast, prostate and colorectal cancers are the most common. For example, ovarian cancer remains the fourth leading cause of cancer-related deaths in women, resulting in more than 26,700 new cases and 14,800 deaths annually in the U.S.
Despite encouraging initial antitumor responses, conventional cytotoxic chemotherapy fails to cure the majority of patients with advanced stage ovarian cancer. With the emergence of drug resistance in refractory tumors, immunologic treatment strategies have been explored.
Monoclonal antibodies have been developed for specific cancer types. HERCEPTIN® (Trastuzumab), RITUXAN® (rituximab), and CAMPATH® (alemtuzumab) have been a clinical and commercial success. But these medicines provide only passive treatment without recruiting constructive participation by the host's immune system. They also leave out what may be the most powerful immune effector mechanism for causing tumor regression: the cytotoxic T lymphocyte (CTL) compartment.
Considerable effort is underway in laboratories all over the world to find an active vaccine that will overcome the natural tolerance to self-antigens, and induce a strong anti-tumor response.
Peptide vaccines have been developed based on tumor associated antigens like carcinoembryonic antigen (CEA) or gp100, sometimes with epitope enhancement to enhance immunogenicity (S. A. Rosenberg et al., Nat. Med. 4:321, 1998). Cytokines, chemokines, or costimulatory molecules have been used as potential adjuvants (J. A Berzofsky et al., Nat. Rev. Immunol. 1:209, 2001; J. D. Ahlers et al., Proc. Nat. Acad. Sci. USA 99:13020, 2002). Active immune response to tumor antigen has also been achieved in cancer patients using anti-idiotype antibody, made to mimic the target antigen while providing further immunogenicity (U.S. Pat. Nos. 5,612,030 and 6,235,280). Nucleic acid vectors based on adenovirus, vaccinia, and avipox encoding such as CEA or prostate specific antigen (PSA) are also in clinical trials (J. L. Marshall et al., J. Clin. Oncol. 18:3964, 2000; M. Z. Zhu et al., Clin. Cancer Res. 6:24, 2000; I. M. Belyakov et al., Proc. Natl. Acad. Sci. USA 96:4512, 1999).
Tumor cell vaccines have also been based on tumor cells taken either from the patient being treated, or from an autologous source bearing a similar profile of tumor antigens. They are genetically modified to express a cytokine like GM-CSF or IL-4 that is thought to recruit the host immune system (J. W. Simons et al., Cancer Res. 59:5160, 1999; R. Soiffer et al., Proc. Natl. Acad. Sci. USA 95:13141, 1998; E. M. Jaffee et al., J. Clin. Oncol. 19:145, 2001; R. Salgia et al., J. Clin. Oncol. 21:624, 2003). Transfected tumor cell vaccines are in late-stage clinical trials for prostate cancer, lung cancer, pancreatic cancer, and leukemia (R. Salgia et al., J. Clin. Oncol. 21:624, 2003; K. M. Hege et al., Lung Cancer 41:S103, 2003).
An improved version of this approach is to isolate the patient's own tumor cells, and combine them with a cell line transfected to express a cytokine like GM-CSF in membranes form (U.S. Pat. No. 6,277,368). The transfected cells recruit the host immune system, which then initiates a CTL response against the tumor cells as bystanders. Another type of cellular vaccine comprises a patient's tumor cells combined with alloactivated T lymphocytes, which again play the role of recruiting the host immune system (U.S. Pat. Nos. 6,136,306; 6,203,787; and 6,207,147).
Because dendritic cells play a central role in presenting tumor antigen to prime the CTL compartment, there has been considerable research interest in autologous dendritic cells as a tumor vaccine (G. Schuler et al., Curr. Opin. Immunol. 15:138, 2003; J. A. Berzofsky et al., J. Clin. Invest. 113:1515, 2004). Clinical trials have been based on dendritic cells from two sources: a) purified DC precursors from peripheral blood (L. Fong & E. G. Engleman, Annu. Rev. Immunol. 15:138, 2003); and b) ex vivo differentiation of DCs from peripheral blood monocytes (F. Sallusto et al., J. Exp. Med. 179, 1109, 1994) or CD34+ hematopoietic progenitor cells (J. Banchereau et al., Cancer Res. 61:6451, 2001; A. Makensen et al., Int. J. Cancer 86:385, 2000).
D. Boczkowski et al. (J. Exp. Med. 184:465, 1996) reported that dendritic cells pulsed with RNA can act as antigen-presenting cells in vitro and in vivo. S. K. Nair et al. (Eur. J. Immunol. 27:589, 1997) reported that antigen-presenting cells pulsed with unfractionated tumor-derived peptides can act as tumor vaccines. F. O. Nestle et al. (Nat. Med. 4:328, 1998) reported vaccination of melanoma patients with peptide- or tumor lysate-pulsed dendritic cells. B. Thurner et al. (J. Exp. Med. 190:16169, 1999) reported vaccination with mage-3A1 peptide-pulsed dendritic cells in Stage IV melanoma. L. Fong et al. (J. Immunol. 167:7150, 2001) described dendritic cell-based xenoantigen vaccination for prostate cancer immunotherapy.
A. Heiser et al. (Cancer Res. 61:338, 2001; J. Immunol. 166:2953, 2001) reported that human dendritic cells transfected with renal tumor RNA stimulate polyclonal T cell responses against antigens expressed by primary and metastatic tumors. C. Milazzo et al. (Blood 101:977, 2002) reported the induction of myeloma-specific cytotoxic T cells using dendritic cells transfected with tumor-derived RNA. Z. Su et al., (Cancer Res. 63:2127, 2003) reported immunological and clinical responses in metastatic renal cancer patients vaccinated with tumor RNA-transfected dendritic cells.
Exosome-based immunotherapy has also recently attracted much attention, since tumor-derived exosomes are a rich source of shared tumor rejection antigens for CTL cross-priming. For immunotherapy, tumor exosomes are usually loaded onto dendritic cells before administering in vivo. Novel approaches to bypass antigen loading onto DC either in vivo or in vitro have been investigated; however, to date, there are no reports regarding the modification of exosomes themselves to improve the antitumor effect of exosome-based immunotherapy. While tumor exosomes appear to be enriched in potential antigenic targets, they also express immunosuppressive and apoptogenic activities. As a result, exosome-based immunotherapy for solid human cancers has exhibited at best marginal statistical success.
Unfortunately, few immunological treatments explored to date have achieved a high frequency of pathologically confirmed complete remissions, due in large part to the presence of an immunosuppressive tumor microenvironment. As such, although immunotherapies have the potential to specifically target and eliminate diseased tissues, including cancers, there is still an unmet need in the art for new immunotherapies that can overcome the immunosuppressive defenses of targeted tissues.